The tagging of materials such as explosives and nitrogen-based fertilizers, or black powders requires taggants which are inert and safe, easy to detect, and inexpensive. Free radical nitroxides are possible taggants, but require isotopic labelingDadding synthetic effort and additional costs of taggant preparation. We present a new strategy of tagging which makes use of free radical precursor compounds as tagging agents. The chemical additives are commercially available or are easily synthesized from commercially available compounds. This has an advantage of low material cost, and does not sacrifice detection or facilitate criminal countermeasures. Detection and analysis are accomplished by EPR/ENDOR spectroscopy. The sensitivity of EPR is at the parts-per-billion level for cavity detection, and potentially at the picomolar level (10-12 mol L-1) or less for loop-gap resonator detection. Examples of implementation of this EPR/ENDOR tagging method will be presented for a class of tert-butyl phenols including 2,4,6-tri-tert-butyl phenol, and for triphenyl chloromethane and other potential taggants.